Unbalance reasons

Main reason for mass unbalance: inhomogeneous mass distribution in the blades and the hub
Main reason for aerodynamic unbalance: pitch angle errors

Impact on the vibration charcteristics

Mass unbalance: Vibrations in the rotor plane (radial) Link bzw. Referenz zu Simulationsfilm, small torsional vibrations around the tower axis
Aerodynamic unbalance: Vibrations perpendicular to the rotor plane (axial) as well as radial and torsional vibrations

Vibration measurement

3 acceleration sensors: nacelle center and rear part for radial vibration, nacelle center for axial vibrations, rotational speed sensor with zero mark

State of the art

Vibration measurements: from axial vibrations we derive the existence of aerodynamic unbalances
Optical methods to determine pitch errors
Removal of aerodynamic unbalances
Measurement of the residual vibrations; mounting of a test weight at one blade; repeated measurement and determination of the mass unbalance

Model based unbalance determination

We create a simple model that allows the computation of axial, radial, and torsional vibrations for given forces and moments from mass and aerodynamic unbalances. Those forces and moments have to be described mathematically, too. This enables us to relate an arbitrary unbalance situation to the corresponding vibrations of the wind turbine in particular at the sensor positions in the nacelle.
To handle this problem is a requirement in order to deal with the inverse problem of determining the unbalances from given measurements at the sensors. This demands special techniques from the Inverse Problem Theory, namely so called regularization methods. The determined unbalances provide quantities to correct the pitch angle as well as to set the necessary balancing masses in the blades.
After the model creation we only need measurements that can be collected by a Condition Monitoring System (CMS), therefore the unbalances determination algorithm can be integrated in a CMS and allows for a continuous unbalance monitoring and the scheduling of the maintenance and the correction procedure.